Transform panel. Quaternion mode. What is the 4L button good for?
Found it out. A quaternion is made of three axis x, y and z. And the value W, which is a calculated value. Normally this calculated value has no lock since it is a mathematical calculated value. It can be used to retrieve the actual rotation around the defined angle.
The 4L button simply allows you to reveal a lock at the W value too. And that's already it for the functionality of this 4L button.